There are demands for a surface lighting device used for a display device to have many characteristics, such as high luminance, low power consumption and thin shape. When improving one of these characteristics, it is liable that other performance has to be more or less sacrificed and it has been very difficult to improve all performances at a time.
To overcome the plurality of challenges, a sheet (lens array sheet) having a lens array formed with a fine concave and convex pattern, such as a prism shape and a Fresnel lens shape, on a surface of a thermoplastic resin mold, such as a transparent plastic, has been increasingly used as an optical part in recent years.
For example, a method of easily attaining high luminance by using a lens array sheet as a light condensing plate for a liquid crystal display device and condensing a display light in an approximately normal direction has become widely used. This method uses a light condensing effect of a prism and is capable of attaining high luminance without increasing an electric power and a thickness of the device, while it has a disadvantage that the luminance drastically declines when being out of the light condensing range.
To solve such a disadvantage, the Japanese Unexamined Patent Publication No. 7-261006 proposes to change a refraction index between one surface and the other surface of the prism. Also, the Japanese Unexamined Patent Publication No. 2000-75102 proposes to use as a material of the light condensing plate an alicyclic structure containing thermoplastic resin having a norbornene structure.
Since an organic electroluminescence material can be used as a thin film, it is an effective material for making a surface lighting device and a display device thinner and lighter. However, light transmittance of a liquid crystal panel is too low to use it as a backlight of a liquid crystal display device, therefore, the light condensing efficiency has to be furthermore improved comparing with that in a conventional backlight using a cold-cathode tube, etc.
However, when using a prism lens having a line shaped surface, which has been conventionally used as a light condensing plate of a liquid crystal substrate, as a light condensing plate of an organic electroluminescence element, there is a disadvantage that the viewing angle becomes narrow.